1. Field of the Invention
The present invention relates generally to direct sequence spread spectrum (code division multiple access) receivers. This invention is particularly concerned with a sync acquisition and tracking technique for direct sequence spread spectrum receivers of cellular mobile communications systems in which transmitted signals are severely affected by Rayleigh fading.
2. Description of the Related Art
Commercial interest in direct sequence spread spectrum communication systems has recently risen due to their potential ability to provide service to more users than is offered by other multiple access techniques. In the cell-site station of the DS/CDMA system, a data symbol is spread by multiplying it with higher frequency orthogonal pseudo-random number (PN) sequences assigned to the cell site as well as with orthogonal Walsh codes assigned to the channel over which the spread signal is transmitted. In order to enable the mobile station to implement synchronous acquisition and tracking operations, a pilot signal is superimposed on the data symbol sequence. At the mobile station, a sliding correlation technique is used to shift the phase timing of a local PN sequence by a predetermined amount each time a correlation is taken between the received and local sequences and determine the correct phase timing for the local sequence when the correlation exceeds some critical value. Once synchronization is established, the phase difference is monitored and maintained to within a fraction of the chip interval. During transmission, the signal undergoes reflections from various land structures, producing a complex pattern of standing waves due to mutual interference of the multipath signals at the mobile station. As a result, the propagation path of the signal exhibits a field intensity distribution which is approximated by the Rayleigh distribution. Thus, the signal experiences a phenomenon called "Rayleigh fading" and the envelope of the signal at the mobile station as well as its phase violently fluctuate.
Under such unfavorable conditions, the sliding correlation technique is not ideal for maintaining phase timing of the local PN sequence in response to the rapidly changing signal variation. More specifically, with prior art DS/CDMA receivers phase shifting is performed at such intervals that if the pilot signal is despread at a trough of Rayleigh fading the correlation is low even if the local clock timing is in sync with the transmitted chip-rate clock. Conversely, if the pilot signal is despread at a peak of Rayleigh fading, the correlation is high even if the local timing is somewhat out of sync with the transmitted clock. Additionally, because of the cost consideration, the degree of precision required for the clock generator for mobile station use is not as high as that of the clock generator used at cell sites. Due to the low degree of precision, the mobile's window interval is not always constant, causing a frequency offset to occur between the local and transmitted clock.